Base it on any common 0.5mm pitch metric thread such as M3 and simply add 4.5mm to the diameters

Martin

Plenty of info here on M fine threads, not in the exact size you need but will give you some idea.

**LINK**

Emgee

To amplify Jason's point, for _any_ thread series (ISO metric, UN, Whitworth, BSCy, PG)* where a particular pitch is used on more than one diameter (so, really anything except BA), the depth of thread is only related to pitch and form (geometry).

In particular, it is independent of diameter. You can cut 1mm pitch on 6mm diameter or on 50mm diameter. The amount you plunge in is the same.

The only place diameter has an influence is when calculating helix angle, which determines the clearance angles required on the cutting tool (the M6 x 1.0 will have a much steeper helix to M50 x 1.0) .

* ISO metric and UN use identical forms, so really this is a list of four possiblities, not five.

I buy the partial form outside threading inserts in the pitch I want. Then cut the outside of the threaded about 0.1 to 0.15 larger in diameter. I then cut the thread until the outside is a little smaller diameter than the nominal of the thread designation. So on a 7.5 X 0.5 pitch thread will make it down to 7.48mm to 7.46mm on diameter.

It doesn't.

The partial profile inserts are not 'topping' inserts.

Every major insert manufacturer spends a lot of time and money producing literature describing their products. The literature stresses that for partial profile inserts, the stock diameter is critical.

See bottom of page 10.

https://usercontent.azureedge.net/Content/UserContent/Documents/034353.pdf

Martin,

If you have a tap, why not just make a die from it? You’ll end up with a perfect match.

Len

Yes, I know you can’t screw matching taps and dies together.  Thought it was common knowledge.  However doesn’t alter the fact it’s easy to make a functional die from a tap.  I’ve done it many times when faced with unusual threads.  Probably in less time than it takes to read ISO2857!

I’m a very practical model engineer and like to make functional parts rather than pontificate about standards and machining to the nearest micron.

Len

On 24 August 2023 at 18:04 Martin Currie Said:

… now trying to machine an external thread for the top of the pen body and I can’t find any information for the max and min diameter for the external threads so I can cut these on the lathe.I’ve got a copy of machinery handbook, but can find any information on this particular thread, I’ve even tried subtracting the pitch from the M7.5, but it’s really slack…

Can anyone help?

 

…

Subtracting the pitch is very much on the right track, but it’s not quite that simple, and the specification makes it look extra complicated, when for our purposes it needn’t be.   Rather than get entangled in Major and Minor diameters, I just calculate the thread depth from the formula:

depth = pitch * 0.75 * cosine(30°)

This simplifies to:

depth = pitch * 0.65

For the pen  0.5 * 0.65 = 0.325mm

This depth I cut into whatever diameter is given by the M number, i,e  7.5mm diameter for M7.5, or a bit under because I rarely want a tight fit.

In practice, don’t cut the thread cut down to calculated depth.  A less than rigid lathe can cause a deeper cut than indicated,   but more important is we are trying to fit with another thread that is probably imperfect.  Though taps and dies in good nick get close to spec under ideal conditions, there is no guarantee the thread they produce is spot on.

The answer is to use one thread to gauge its fit to the other rather than cut directly to the theoretical measurement.  Compare, don’t measure.

In this pen example:

• the tapped interior thread becomes the master.  We know because it was cut with a tap, that its close to specification, but how accurately or not doesn’t matter.
• On the lathe, starting with a 7.5mm diameter, or perhaps very slightly over, cut a 0.5mm pitch thread to a little under theoretical, say 0.28mm deep rather than 0.325mm.  Do this with a series of shallow cuts.
• Then try to screw the interior thread on to it.  Shouldn’t go yet, because not enough metal has been removed, but it will obviously be about right unless a mistake has been made.
• Take another shallow cut – whilst learning don’t rush to remove metal quickly.  I’d take off 0.02mm, and try again.  Repeat until the two threads just start to engage, then do several passes without advancing the cutter.  Stop and clean up the threads, which are probably slightly burred.   A light touch with a fine file, or Scotchbrite is usually enough to tidy up the crests.  This should deliver a very tight fit.

Often as not, a very tight fit isn’t wanted, in which case the operator can cut a little deeper until the fit feels suitably loose.

Mostly done by feel, avoiding the need for difficult thread measurements, and tools in tip top condition.   Wouldn’t do for production work, where parts need to be interchangeable, but the method is ideal for home workshops, where the goal is for two parts of one pen to screw together satisfactorily.

One booby trap.   Lathe dials are often, but not always, graduated in metal removed from diameter, which is 2x depth of cut.   So when cutting threads, be careful to understand the dial!

Recommended: Tubal Cain’s “Model Engineer’s Handbook”   In the latest edition (3rd), there’s a table of lathe infeeds for all the major threads.   Not quite the same as me, I say 0.65 for all metric interior and exterior threads, whereas the book gives different values for rounded and sharp threads in both exterior and interior.  We’re in the same ball-park: TC says 0.625 x Pitch for Rounded form bolts and 0.72 x Pitch for sharp form.  (Sharp assumes a V-cutter, whereas Round needs a shaped cutter, such as a carbide threading insert.)  My answer suits my simple thread fitting needs, Tubal Cain takes it to the next level, if needed.

Dave